The present invention relates to an electrophotographic recording material comprising an electrically conducting base layer and a photoconductive system provided thereon, which system comprises an optional insulating intermediate layer, a layer containing a chargecarrier generating compound, and a layer containing a charge-transporting compound mixed with a binder, as well as with sensitizers, acceptors and conventional additives.
The use of sensitizers and acceptors in photoconductive systems, particularly in a photoconductive double-layer arrangement, is known (see German patent No. 1,127,218, corresponding to U.S. Pat. No. 3,287,123; German Auslegeschrift No. 1,572,347 corresponding to U.S. Pat. No. 3,484,237; and German patent No. 2,220,408, corresponding to U.S. Pat. No. 3,973,959).
In German Offenlegungsschrift No. 3,331,592, corresponding to British patent No. 2,130,597, photoconductive layers are described that contain, for example, methyl violet or rhodamine B as sensitizers and, for example, tetracyanoethylene or chloranil as acceptors. With the charge-transporting compounds, they often form colored charge-transfer complexes which may result in an undesirable increase in conductivity in the dark and unstable charging characteristics.
In European patent application No. 0 069 397, photoconductive double layers are described which contain dicyanovinyl compounds as acceptors in the transport layer. These compounds and their charge-transfer complexes have interfering inherent absorption in the blue spectral region up to about 475 nm. In addition, they can only be prepared in a two-stage synthesis step.
European patent No. 0 058 084 describes a sensitizer of the nitrophthalic anhydride type which is suitable for activating monodisperse photoconductive films containing phthalocyanine derivatives and polyester resins.
It is further known from German patent No. 2,734,288, corresponding to U.S. Pat. No. 4,220,697, that certain organic photoconductors and pigments in combination with cellulose nitrates yield highly sensitive photoconductive double layers. But a disadvantage in this connection is a higher pre-exposure sensitivity.
The known publications regarding photoconductive double layer arrangements disclose materials containing various pigments that impart good photosensitivity. The residual discharge properties of these materials, and their occasionally encountered, unstable cyclic characteristics are still unsatisfactory. The residual discharge properties and unstable characteristics which are also binder-dependent, involve variation in charge acceptance and increasing residual charge in cyclic copier operation.